1. Field of the Invention
This invention relates generally to electro-optical modulation devices and their methods and use, and more particularly to electro-optical modulation devices formed on a single chip, and their methods of use.
2. Description of the Related Art
Lasers are widely used today in fiber and free space segments for high data rate communication links, remote sensing applications (LIDAR) and more. In these applications the light signal is modulated, usually using electro-optical modulators.
In optical communications the modulation scheme commonly used is On-Off Keying (OOK), see FIG. 2a, where only the power of the light is modulated. Alternative modulation schemes include Phase Shift Keying (PSK), where the data is encoded in the phase of the signal. In RF communications more advanced modulation schemes are used, such as QPSK (quadrature phase shift keying) and QAM (quadrature amplitude modulation) see FIG. 2b. By using such communication schemes, for example, in optical communication systems, the capacity and link performance can be greatly enhanced in comparison with the direct detection schemes. In LIDAR, which is the extension of radar to the optical domain, the required pulse shaping can be achieved, such as chirped pulses, Barker coding, etc.
For these applications and others, the light should be modulated both in amplitude and phase, essentially with a complex modulation signal. The present invention is an integrated electro-optical modulator capable of modulating a light signal with an arbitrary complex signal.
The previous experiments in this field were based on un-integrated solutions (viz. separate electro-optical components connected by fibers). This practice reduces performance and increases cost.
Such modulating formats as PSK (for example, BPSK and QPSK) were used mostly in the coherent communication systems (see, for example, T. G. Hodgkinson, “Demodulation of Optical DPSK using in-phase and quadrature detection”, Electronics Letters, Vol. 21, No19, pp. 867–868, 1985). The majority of the work in this field was made by implementing non-integrated solutions, i.e. various optical components such as amplitude and phase modulators connected by optical fibers. Such communication schemes were abandoned in the late 1980's and are still not implemented due to their complexity and high cost.
Some examples of monolithic integration of optical modulators have different implementations and/or are still far from being implemented in practical optical systems (see, for example, S. Shimotsu at. al. “Single Side-Band Modulation Performance of a LiNbO3 Integrated Modulator Consisting of Four-Phase Modulator Waveguides”, IEEE Photon. Tech. Letters, Vol. 13, No. 4, pp. 364–366, 2001 or R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wate, J. Hughes, P. A. Jerram, and N. J. Parsons, “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration”, Proceedings OFC-2002, March 2002, Anaheim, Calif., post-deadline paper).
Optical devices including X-cut LiNbO3 have been described in, for example, U.S. published application no. 2001/0007601, filed Jul. 12, 2001, and U.S. Pat. No. 5,416,859, issued May, 16, 1995. The U.S. Pat. No. 5,526,448, filed Jun. 11, 1996 discloses the optical waveguide modulator with a reduced DC drift. The foregoing published application and patent are incorporated by reference to the extent necessary to understand the present invention.
Optical devices currently available are based on non-integrated and/or semi-integrated solutions, i.e. optical fibers or optical fiber-based components were used for connecting of various electro-optical components and/or splitting/combining the optical signals. There are no completely planar integrated solutions for the device that is capable to provide an arbitrary modulating format (phase and/or amplitude modulation)..
Accordingly, there is a need for integrated monolithic devices that provide modulation of the input signal in phase and/or amplitude domain. There is a further need for integrated monolithic devices that provide quadrature phase shift keying (BPSK and/or QPSK) or quadrature amplitude modulation (QAM) by use of a single, monolithically integrated device. There is a yet a further need for improved devices that is re-applicable for BPSK and/or QPSK communication systems, controlled chirp or Barker coding for LADAR as well as other remote sensing applications.